In the context of this invention the term “document of value” refers to bank notes, checks, share certificates, tokens, ID documents, credit cards, passports and other documents as well as labels, seals, packagings or other elements for the product protection.
The protection of documents of value against forgery by means of luminescent substances has already been known for a long time. The use of transition metals and rare earth metals as luminescent ions has already been discussed. Such ions have the advantage that they, after appropriate excitation, show one or a plurality of characteristic narrow-band luminescences which facilitate a reliable detection and the delimitation against other spectra. Combinations of transition metals and/or rare earth metals have also been already discussed. Such substances have the advantage that, in addition to the above-mentioned luminescences, so-called energy transfer processes are observed which can lead to more complicated spectra. In such energy transfer processes an ion can transfer its energy to another ion and then the spectra can consist of a plurality of narrow-band lines which are characteristic for the two ions.
An example of a known luminescent substance is Al2O3 doped with Cr3+. This substance is also referred to as ruby. After excitation in the visible spectral range the ruby luminesces in a narrow-band fashion.
DE 198 04 021 A1 describes a document of value with at least one authenticity feature in the form of a luminescent substance on the basis of doped host lattices. These are authenticity features, which are based on an energy transfer step from Cr3+ to a rare earth ion. The luminescence then is observed with the rare earth ions.
EP 1 370 424 B1 describes a printed document of value having at least one authenticity feature in the form of a luminescent substance on the basis of host lattices which are doped with ions of the (3d)2 electron configuration.
But luminescent substances, which consist of a host lattice doped with ions and are suitable for the protection of documents of value, in general are limited in their number.
Substances luminescent in a narrow-band fashion which are based on transition metals and are suitable for the protection of documents of value are especially limited in their number. Moreover, they have the disadvantage that these luminescences have a long lifetime.
Luminescent substances which are based on rare earth metals have the disadvantage that the spectral position of the narrow-band luminescences can be influenced only very poorly by the host lattice. Moreover, most of the substances can be excited only very inefficiently.